In automotive gauge systems, sensor resistors are located at the measurement point and one end is locally grounded. The other end is connected by a wire to the remote readout device which is also grounded at its own location. For example, to sense engine temperature the sensor resistor is located in thermal contact with the engine block and one resistor terminal is thereby grounded to the engine book. The other resistor terminal is connected via a wire to the gauge. The gauge is powered by the vehicle battery and grounded to the frame of the vehicle. Since the sensor ground is remote from the gauge ground their actual potentials can be different. It has been found that various ground points throughout the vehicle can develop offset potentials at several volts. Since the offset is in series with the sensor resistor a simple d-c gauge circuit will include an offset error.
FIG. 1 is a schematic diagram of a typical prior art fuel gauge circuit. It is powered From a V.sub.CC supply, provided by the vehicle battery, connected+to terminal 10 and-to ground terminal 11. The gauge meter itself is 60.degree. to 70.degree. deflection air core coil driven structure. Coil 12 comprises a pair of meter coils which typically have a 260 ohm resistance each. Resistor 13 shunts one element of coil 12 and sensor resistor 14 shunts the other coil. Resistor 14 typically has a value of 1 to 88 ohms as a function of the fuel level. Ground 15 would be a direct connection to the fuel tank which is a vehicle ground. Ground 16 represents the gauge ground which is typically located upon the vehicle dashboard. The ground offset potential 17 represents the difference in potential that exists between the gas tank and the dashboard. It can be seen that the offset potential is in series with the sense resistor 14 as far as the gauge circuit is concerned.
FIG. 2 is a graph depicting the typical performance of the gauge of FIG. 1 The horizontal scale represents the value of resistor 14 and the vertial scale represents the deflection of the meter which consists essentially of coil 12. Curve 18 is a linear plot extending over the span of the gauge. Curve 19 is an actual plot of the meter deflection as a function of resistance. It can be seen that there is substantial nonlinearity. Curve 20 shows the gauge performance in the presence of a 2-volt offset potential 17. The offset error Is in excess of 10 degrees at the low resistance portion and in excess of 5 degrees at the high resistance end. It can be seen that gauge performance is poor at least due to nonlinearity errors and the presence of an offset potential can. Further increase the error. Such performance has been considered acceptable. For the conventional narrow angle (60 degree) gauges. It is to be understood that in the prior art it was well known that if the gauge circuit of FIG. 1 is provided with a constant current input supply (rather than constant voltage as shown) the linearity will be greatly improved. However, it has been felt that the added cose will not justify the improved linearity. Accordingly, the FIG. 1 circuit has been widely used as shown.
In recent years wide angle air core meters have become available. For example, the Faria Company, of Uncasville, CT., has produced a Model MT203 air core moving magnet indicator. It consists of a pair of 225 ohm air core coils mounted in quadrature relationship. A pointer attached to a moving magnet can be driven over a 305-degree range at typically +2% accuracy. The LM1819 integrated circuit includes a function generator that develops suitable coil currents in response to the output of a Norton amplifier that receives a current input. This IC device is produced by National Semiconductor Corporation to drive the coils in wide-angle meters. This integrated circuit is listed on pages 5-117 to 5-124 of LINEAR DATABOOK 3.COPYRGT.1987, by National Semiconductor Corporation. The teaching in this publication is incorporated herein by reference. As shown in the databook, one application calls for a current input of 0-100 microamperes which will produce a pair of output currents that, when combined in the air more coils of a wide angle meter, will drive the meter pointer over a 0 to 270 degree deflection. The device can be made voltage responsive by coupling a voltage-to-current converter to the control current input terminal.
The LM1819, when driving a wide angle gauge indicator, provides good linearity, but, in the typical automotive environment, will be subject to offset potentials. It would be desirable to eliminate offset potential response in a wide angle qauqe driver.